The present invention relates to an apparatus for heat treating a synthetic yarn, such as polyester or polyamide, and especially relates to an apparatus for heat treating a synthetic yarn which apparatus is installed in a textile machine for false-twisting or for drawing and false-twisting the synthetic yarn at a high speed of at least 400 m/min. The present invention relates to a heat treating apparatus which is suitable for a so called first heater for heat setting twists which are imparted to a synthetic yarn by a false-twisting device disposed downstream of the heat treating apparatus and which run back along the synthetic yarn.